The present application is related to an improved method of forming fine and ultrafine powders and nanopowders. More specifically, the present invention is related to the formation of fine and ultrafine powders and nanopowders through complexometric precursors formed on bubble surfaces.
Our present society is advancing very rapidly in new technologies especially in the areas of biotechnology, medicine, electronics, pharmaceuticals and energy. These require significant improvements in raw material processing and in the production of high performance products of advanced chemical formulations without compromising cost relative to commercial scale-up for industrial production (FIG. 1). Thus, this requires a combination of structure-processing-property correlations that will lead to specialized high performance materials in order to sustain these modern technically demanding criteria.
Starting with a desired specific application, the process must be tailored to obtain the characteristics, both physical and chemical, in order to meet the end performance result. It is imperative to uniquely combine both well-established properties of the compounds and/or raw materials with the new, unique, unusual or desirable properties of the advanced materials. For example, traditional ceramics are well-known to be electrical insulators yet it is possible to utilize this property such that the special ceramics will provide high thermal conductivity allowing their use as heat sinks in substrates for microelectronics. Ceramic composites of inorganic glass fibers and plastics have been used for thermal and sound insulation traditionally but now are also used as optical fibers replacing the traditional copper wire. Ceramic engines replacing the traditional steel engines can withstand higher temperatures and will burn energy more effectively. This requires that the ceramics used for engine manufacture be of very fine particles such that strength and toughness to withstand the elevated temperatures and ruggedness required for these applications. Furthermore, nanosize powders when fabricated into the ceramic parts for these vehicles will be more dense, have less defects, and can be fabricated in thinner and smaller, lightweight sizes for practical use.
Increased energy consumption today necessitates discovery of new resources but also improvement in current materials to satisfy the energy infrastructure such as solar cells, fuel cells, biofuels, and rechargeable batteries. For example, the lithium ion battery that has been in use in consumer electronic devices but is now commanding a significant role in larger transport vehicles. These alternative energy resources must be more practical, and price competitive with fossil fuels, for wider acceptability in high-performance applications. As a consequence, sophisticated devices require specially designed microstructures that will enhance the physical and chemical properties of the materials utilized. Often, these materials are more expensive to produce on an industrial scale. Furthermore, these specialty powdered materials such as oxides, phosphates, silicates and the like, require not only a nanosize material but also a narrow particle size distribution with high porosity, high surface area and other characteristics to achieve enhanced performance. For instance, a nanostructured lithium cathode powder for the lithium ion battery would be expected to have improved mass and charge transport due to shorter diffusion paths and higher amount of active sites resulting from its finer smaller particle size. However, this added cost for the added value may not be acceptable to the end consumer resulting in reduced sales.
Other challenges are medical applications such as the use of calcium phosphate for bone substitution. While several calcium phosphate powders are available in the market, the requirements of less than one micron discrete particles as described in U.S. Pat. No. 8,329,762 B2 are important for making a biocompatible synthetic bone. U.S. Pat. No. 5,714,103 describes bone implants based on calcium phosphate hydraulic cements, called CHPCs, made of a succession of stacked layers with a macroporous architecture mimicking the natural porosity of spongious bone. This medical field would definitely benefit from improved powders with better performance and lower cost. Another example is a dermal patch wherein the pharmaceutical drug is released to the body. Both dermal patch and drug material combined would be more compatible if their particle sizes were nanosize with narrow particle size distribution. Nanopowders can also significantly impact high performance dental applications, for example, such as teeth filling materials as well as enamel coating materials to aesthetically enhance and strengthen the tooth structure. In order to widen the usage of nanomaterials in the medical field, both cost and performance value should be compatible to both producer and end-user.
Distinctive characteristics clearly differentiate between advanced materials and traditional materials in several aspects, notably in raw materials, processing, chemical and physical characteristics, novel applications and specialized markets. Conventional powder processes are made without strict chemical control and are generally made from grinding and segregating naturally occurring materials through physical means. These result in neither ultrapure nor ultrahomogeneous particles such that fabrication of a product using such heterogeneous and impure substances gives grain boundary impurities that may reduce mechanical strength or optical deformations and other limitations. Chemical processing solves this problem by controlling the composition of the powder at the molecular level to achieve a special ultrastructure for the preferred performance application. Specialized properties such as conductivity, electrochemical capacity, optical clarity, dielectric value, magnetic strength, toughness and strength are met only with specialized processing methods to control microstructure. However, these demands necessitate an economically commercial viable process for large scale production. The dual requirements of cost and performance must be met to successfully commercialize these advanced materials.
A significant improvement in available raw materials is needed to meet many objectives. One objective is high purity, no longer 90% but >99% and even 99.999%, which entails chemical processing to remove undesirable impurities that affect performance. Another objective is particle size which preferably has a narrow, homogeneous particle size distribution with finer particle sizes of no longer 50 microns but 1 micron and preferably, nanosize. The addition of dopants which are deemed to enhance the specialized properties, like electronic conductivity and others, must be homogeneously distributed but also preferably distributed on the surface of the powder in some applications. Cobalt, aluminum and gadolinium are suitable dopants. Aluminum and gadolinium are particularly suitable.
Innovations in processing these advanced materials to the final product are also necessary. As such, combinations of different processing techniques are often utilized. For example, inorganic powders have been usually made by traditional ceramics like solid state sintering. However, the resulting powder obtained by this method alone generally has a wider and larger particle size distribution. To obtain a homogeneous nanosize distribution, several grinding and milling steps have been employed. The generic types are ball mills, rod mils, vibratory mills, attrition mills, and jet mills. Disadvantages of these methods include energy and labor intensive production cycles and possibility of contamination from grinding balls utilized. Defects in the microstructure also occur causing degradation in the required performance targets. Chemical vapor deposition, emulsion evaporation, precipitation methods, hydrothermal synthesis, sol-gel, precipitation, spray drying, spray pyrolysis and freeze drying are some of the other methods used for these types of preparations, each with advantages and disadvantages.
The technical drivers today call for particles less than one micron, and even to less than 100 nanometers. To date, the significance of the initial powder synthesis steps have been overlooked but these initial reactions clearly define the final finished powder microstructure and also determines scalability controls and finally, cost and performance. Careful selection of the starting reactants and the media-solid, liquid or gas-plays a unique role in the formulation of low cost, high performance powders.
An example is the formation of colloidal consolidated structures by initial dispersion of particles in a liquid medium. When the particle concentration is low, dispersed colloidal suspensions can be used to eliminate flow units larger than a certain size through sedimentation or classification. The surface chemistry of the particles can be modified through the adsorption of surfactants. The mixing of multiphase systems can be achieved at the scale of the primary particle size. Once the desired modifications are achieved, the transition from dispersed to consolidated structure is accomplished by either increasing the particle-particle attraction forces, such as by flocculation, or by increasing the solids content of the suspension for forced flocculation. This whole process results in going from a fluid state (“slip”) to a solid phase transition (“cast”). While this has been found to occur in the micron to sub-micron size range, highly concentrated suspensions with nanometer size particles have not been as successful. Thus, some innovation is needed in traditional colloidal techniques in order to achieve nanosize powders.
Such nanoparticles possess crystalline properties and other nanoscale features that dramatically result in unique mechanical, magnetic, thermal, optical, biological, chemical and electrical properties. Considerable growth is expected in all these markets. Therefore, achievement of an economically viable industrial production of these specialized materials entails innovations in conventional processing techniques and distinct improvements in present industrial equipment.
Traditionally, powders are made using a solid state route. By this method, the raw materials are ground and milled to the same size and with a narrow size distribution, blended and fired to obtain the final product as shown:A solid+B solid→C solid product
In U.S. Pat. No. 6,277,521 B1, Manev et al. describe the preparation of lithium metal oxides such as LiNi1-xCoyMαM′βO2 where M is Ti or Zr and M′ is Mg, Ca, Sr, Ba, and combinations thereof. To prepare LiNio.?Coo.zTio.osMgo.osOz, stoichiometric amounts off LiOH, NiO, Co3O4, TiO2 and Mg(OH)2 are weighed, mixed and fired for 10 hours at 550° C. followed for an additional 10 hours at 800° C. Milling after the firing step is done to produce the fine powders of micron size. Furthermore, to obtain a narrow particle distribution, sizing selection is also done in line with the milling step. Larger size fractions are then re-milled.
One of the problems with obtaining nanopowders via the solid state method is the considerable milling process that can be time and labor intensive. The quality of the final product is a function of time, temperature and milling energy. Achieving nanometer grain sizes of narrow size distribution requires relatively long processing times in smaller batches, not just for the final sintered product but also for the starting materials, as these materials should have particle sizes within the same distribution for them to blend more homogeneously in order to have the right stoichiometry in the final product. Hence, it may become necessary to correct the stoichiometries of the final product after firing by reblending additional starting raw materials and then refiring. As a result, successive calcinations make the processing time longer and more energy intensive which increases production cost. Production of nanopowders by mechanical attrition is a structural decomposition of the coarser grains by severe plastic deformation instead of by controlled cluster assembly that yields not only the right particle size and the required homogeneous narrow size distribution but also significant nanostructures or microstructures needed for effective performance benchmarks. As such, some higher performance standards required for specialized applications are not attained. C. C. Koch addresses these issues in his article “Synthesis of Nanostructured Materials by Mechanical Milling: Problems and Opportunities”, Nanostructured Materials, Vol. 9, pp 13-22, 1997.
Obtaining fine powders and nanopowders by milling has improved with modern grinding machines such as stirred ball mills and vibration mills for wet grinding or jet mills for dry grinding processes. However, achieving a narrow particle size distribution still remains a difficult task today. Classifiers have to be integrated with the milling system and this repetitive sizing and milling procedures increase the processing time in making fine powders and even much longer for nanopowders. Another drawback is potential contamination of the final product from the milling media used. U.S. Pat. No. 7,578,457 B2, to R. Dobbs uses grinding media, ranging in size from 0.5 micron to 100 mm in diameter, formed from a multi-carbide material consisting of two or more carbide forming elements and carbon. These elements are selected from the group consisting of Cr, Hf, Nb, Ta, Ti, W, Mo, V, Zr. In US Patent Application No. 2009/0212267 A1, a method for making small particles for use as electrodes comprises using a first particle precursor and a second particle precursor, milling each of these precursors to an average size of less than 100 nm before reacting to at least 500° C. As an example, to make lithium iron phosphate, one precursor is aluminum nitrate, ammonium dihydrogen phosphate and the like and the other precursor is lithium carbonate, lithium dihydrogen phosphate and the like. In US Patent Application No. 2008/0280141 A1, grinding media with density greater than 8 g/mL and media size from 75-150 microns was specially made for the desired nanosize specification and the hardness of the powder to be milled. The premise is that finer, smaller size, specialized grinding media can deliver the preferred nanosize particles. Time and energy consumption are high using this modified solid state route to nanopowders. Moreover, after milling, the grinding media and the nanopowders must be separated. Since nanopowders are a health risk if inhaled, the separation will have to be done under wet conditions. The wet powders will then have to be dried again which adds to the number of processing steps.
Chemical vapor deposition, physical vapor deposition, plasma synthesis are all synthesis of powders in the gas phase. In this process, the starting raw materials are vaporized in the gas phase then collected in a cooling step on a chosen substrate. Controlled nucleation yields excellent powders that easily meet the rigorous requirements for specialized applications but the cost of the energy source and the equipment required for this method can significantly impact the final cost of the powder. More information on these processes is discussed by H. H. Hahn in “Gas Phase Synthesis of Nanocrystalline Materials, “Nanostructured Materials, Vol. 9, pp 3-12, 1997. Powders for the semiconductor industry are usually made by this type of processing.
In U.S. Pat. No. 8,147,793 B2, S. Put et al. disclose a method of preparing nano-sized metal bearing powders and doped powders by using a non-volatile metal bearing precursor and dispersing this precursor in a hot gas stream. This hot gas stream may be generated by a flame burner or a DC plasma arc with nitrogen as a plasma gas, for example. Thus, coarse size ZnO powder that is injected is reduced to Zn vapor. When air is introduced, Zn is oxidized to ZnO with nano-size particles.
Among the wet solution methods for fine powder synthesis are precipitation, sol-gel, and variants of these using complexing agents, emulsifiers and/or surfactants. In WO 2010/042434 A2, Venkatachalam et al. describe a co-precipitation process involving metal hydroxides and sol-gel approaches for the preparation of Li1+xNiαMnβCoγMoτO2−αFz where M is Mg, Zn, Al, Ga, B, Zr, Ca, Ce, Ti, Nb or combinations thereof. In one example cited, stoichiometric amounts of nickel acetate, cobalt acetate, and manganese acetate were dissolved in distilled water to form a mixed metal acetate solution under oxygen-free atmosphere. This mixed metal acetate solution was added to a stirred solution of lithium hydroxide to precipitate the mixed metal hydroxides. After filtration, washing to remove residual Li and base, and drying under nitrogen atmosphere, the mixed metal hydroxides were mixed with the appropriate amount of lithium hydroxide powder in a jar mill, double planetary mixer or a dry powder mixer. The mixed powders were calcined at 400° C. for 8 hours in air, cooling, additional mixing, homogenizing in the mill or mixer, and then recalcined at 900° C. for 12 hours to form the final product Li1.2Ni0.175Co0.10Mn0.525O2. The total time from start to finish for their method is 20 hours for the calcination step alone plus the cooling time, the times for the initial mixed metal hydroxide precipitation, milling and blending to homogenize, and the filtration and washing steps. All these process steps add up to a calcination time of 20 hours excluding the cooling time for the furnace and the time from the other processing steps which will have a combined total of at least 30 hours or more. Furthermore, in their process, the second part after the co-precipitation is a solid state method since the mixed metal hydroxides and the lithium hydroxides are mixed and then fired. The final calcined powder size obtained from a solid state route is usually in the micron size range which will entail additional intensive milling to reduce the particles to a homogeneous narrow size distribution of nanopowders. This processing has numerous steps to obtain the final product which can impact large scale production costs.
Another example of co-precipitation is described in U.S. Pat. No. 6,241,959 81. Nitrates of nickel, cobalt and magnesium were mixed in a mole ratio of 0.79:0.19:0.02 and dissolved in solution. Aqueous ammonia was added to precipitate the hydroxides and the pH was further adjusted using 6M NaOH till pH 11. After 6 hours of addition time, the Ni—Co composite hydroxide was separated. Lithium hydroxide was mixed with this Ni—Co hydroxide and heated to 400° C. and maintained at this temperature for 6 hours. After cooling, the product was then reheated to 750° C. for 16 hours. The battery cycling test was done at a low C rate of 0.2 C. Discharge capacity was 160 mAh/g. Only 30 cycles were shown. Note that the coprecipitation process is only for the Ni—Co hydroxides. The second part of this process is a solid state synthesis where the starting raw materials, Ni—Co hydroxide and the lithium hydroxide are mixed and then fired. The addition of NaOH to raise the pH to 11 as well as provide a source of hydroxide ions would leave residual Na ions in the final product unless the excess Na+ is washed off. This excess Na+ will affect the purity of the material and have some deleterious effect in the battery performance. The total process time is 6 hours addition time for the co-precipitation step, 22 total hours for the holding time at the two heating steps and additional time for the other steps of cooling, separating, mixing and others which sums up to at least 40 hours of processing time.
Sol-gel synthesis is a variant of the precipitation method. This involves hydrolysis followed by condensation to form uniform fine powders. The raw materials are expensive and the reaction is slow since the hydrolysis-condensation reactions must be carefully controlled. Alkoxides are usually the choice and these are also air sensitive; thus requiring the reactions to be under controlled atmosphere.
Hydrothermal synthesis has also been used to prepare these powders. This involves crystallization of aqueous solutions at high temperature and high pressures. An example of this process is disclosed in US Patent Publication No. 2010/0227221 A1. A lithium metal composite oxide was prepared by mixing an aqueous solution of one or more transition metal cations with an alkalifying agent and another lithium compound to precipitate the hydroxides. Water is then added to this mixture under supercritical or subcritical conditions, dried then followed by calcining and granulating then another calcining step to synthesize the lithium metal oxide. The water under supercritical or subcritical conditions has a pressure of 180-550 bar and a temperature of 200-700° C.
The use of agents like emulsifiers, surfactants, and complexing agents to form nanosize powders has been demonstrated. In microemulsion methods, inorganic reactions are confined to aqueous domains called water-in-oil or surfactant/water/oil combination. A problem is separation of the product particle from the oil since filtration of a nanosize particle is difficult. Reaction times are long. Residual oil and surfactant that remain after the separation still have to be removed by other means such as heating. As a result, the batch sizes are small.
A variety of structures are formed by the surfactant with another particle dispersed in solution. Micelles are formed at high concentrations of the surfactant and the micelle diameter is determined by the length of the surfactant chain which can be from 20-300 angstroms. U.S. Pat. No. 6,752,979 B1 describes a way of making metal oxide particles with nano-sized grains using surfactants. A concentrated aqueous solution of at least one or more metal cations of at least 90% of its solubility is mixed with surfactant to form micelles at a given temperature. Optionally, this micellar liquid forms a gel. This mixture is heated to form the metal oxide and remove the surfactant. A disadvantage is the long heat treatment times.
U.S. Pat. No. 6,383,285 B1 discloses a method for making cathode materials for lithium ion batteries using a lithium salt, a transition metal salt, and a complexing agent in water then removing water by spray-drying to form a precursor. These complexing agents were citric acid, oxalic acid, malonic acid, tartaric acid, maleic acid and succinic acid. The use of these agents increases the processing cost of the product. The precursor is formed from the lithium, transition metal and the complexing agent after spray drying. Battery capacities were only given for the first cycle. The C-rate was not defined. For electric vehicle applications, lithium ion battery performance at high C-rate for many cycles is an important criterion.
A method for making lithium vanadium phosphate was described in US Patent Publication No. 2009/0148377 A1. A phosphate ion source, a lithium compound, V2O5, a polymeric material, solvent, and a source of carbon or organic material were mixed to form a slurry. This wet blended slurry was then spray dried to form a precursor which was then milled, compacted, pre-baked and calcined for about 8 hours at 900° C. The particle size after spray drying was about 50-100 microns. The final product was milled to 20 microns using a fluidized bed jet mill.
Nanosize Li4Ti5O12 was prepared by preparing this lithium titanate as a first size between 5 nm to 2000 nm as described in U.S. Pat. No. 6,890,510 82 from a blend of titanium and lithium, evaporating and calcining this blend, milling this powder to a finer size, spray drying then refiring this lithium titanate, then milling again. There are several milling and firing sequences in this process to obtain the nanosize desired which increase the number of processing steps which consequently increases the cost of processing.
In spite of the significant efforts and advances in the art the art is still lacking a method of preparing small, nanoparticles with narrow particle size distributions, controllable particle size and which can be prepared economically.